The present invention relates to a piston for internal combustion engines, particularly for high-speed, high-duty diesel engines and a method of making same, which comprises an upper part and a lower part and is formed adjacent to the interface between these parts, with one or more cooling passages and in which the upper part consists of forged steel and is formed on its underside with ribs bearing on opposite surfaces of the lower part.
Three types of pistons are generally used at present in internal-combustion engines:
(a) Cast or forged, integral pistons of light alloys; PA0 (b) Composite pistons having an upper part made of ferrous material and a lower part made of a light alloy; PA0 (c) Integral pistons cast of ferrous material.
In the present four-stroke cycle diesel engines,
the use of integral light alloy pistons is decreasing because light alloys when used alone can no longer meet the increasing requirements for modern engines as regards a continuous increase in power, poorer fuel qualities and longer life. For this reason the pistons of the two other types will mainly be used in the engines being developed for the 1980's.
In such engines, the higher combustion pressures and combustion temperatures must be matched with the existing technological possibilities which depend on the material and its processing and last, but not least, on economic considerations. These facts usually result in agreement with the engine manufacturer in a compromise by which an optimum is achieved as regards expenditure and function, as is usual in engineering. In view of the high factor of safety and the long life which are required, the cooled composite piston generally meets the requirements at the present state of the art.
The cooled composite piston relies on a separation of materials with a view to the different functions to be performed by different parts of the piston, each material is used in such a manner that only its advantages are obtained, as far as possible. Forged steel, such as steel having the designation 42 CrMo 4 V, is used for the upper part, which is formed with the combustion chamber recess and the piston ring grooves and is subjected to high thermal and mechanical stresses and to abrasion. The lower part comprising the skirt, which is guided in the cylinder, and the piston pin bosses consists in the presently standard piston of a forged eutectic aluminum-silicon alloy so that the low friction and the low specific gravity of aluminum are utilized. The use of screws to connect the two parts has been found to be most desirable. A critical aspect of such structure resides in the fact that aluminum has a higher coefficient of expansion and a lower modulus of elasticity than steel and that its permissible load decreases in the course of operation.
More recently, nodular iron, which has a high strength and elongation, has been used for integral pistons cast from ferrous material. Because nodular iron has a low thermal conductivity and the cooling passage is specially designed, almost all heat which enters the piston is dissipated in the upper one-fourth of the piston so that the skirt and the piston pin bosses assume the temperature if the lubricating oil.
For this reason, it is no longer necessary to design the skirt of pistons of nodular iron with the convex surfaces and complicated oval shapes which are known and required in light alloy pistons.
When the piston heads are formed with unfavorably designed, undercut combustion chambers or when the piston is operated under severe conditions, exposed surface portions of the piston head may be heated above 450.degree. C. so that ferrite will form in the nodular iron and its high strength will be decreased.